1. Field of the Invention
The present invention relates to an audio signal recording apparatus and method for use with video tape recorders (VTR's) such as those which record high definition television (HDTV) signals.
2. Description of the Prior Art
Some VTR's are designed to record and reproduce HDTV signals which have a wideband and a high frequency. Typically these VTR's record an HDTV signal per unit time (e.g., over one field) by dividing the signal into a plurality of segments on a plurality of channels. As a result, the frequency of the recorded signal is lowered and the frequency band is reduced.
FIG. 7 is a typical rotating head device for use with the above-mentioned VTR's. In FIG. 7, an A channel head 1A having a first azimuth position and a B channel head 1B having a second azimuth position, which is different from the first azimuth position, are located so as to have the same angular position with respect to a rotating drum 3, that is, the heads 1A and 1B are one track width apart along the axis of rotation. Likewise, another A channel head 2A having the first azimuth position and another B channel head 2B having the second azimuth position are located 180 degrees away from heads 1A and 1B, in which the heads 2A and 2B are also one track width apart along the axis of rotation of the drum 3. A magnetic tape 4 is wound obliquely around a 180 degree-plus angular portion of the drum 3. The rotating heads 1A, 1B, 2A and 2B are rotated at a rotating speed of 60 Hz.
In the above-described rotating head device, each pair of rotating heads, that is, 1A, 1B and 2A, 2B, alternately records the HDTV signals to the oblique recording tracks on the magnetic tape 4 shown in FIG. 8. Thus each pair of heads records the signals over a tape contact segment corresponding to that obtained by rotating the drum 3 through a 180 degree angle.
More specifically, during the first half of one rotation of the rotating drum 3, that is, a 180 degree rotation, the rotating heads 1A and 1B contact the tape 4. During this half rotation, the rotating heads 1A and 1B simultaneously form an A channel track TA1 and a B channel track TB1 respectively. During the second half of the rotation or the second 180-degree rotation of the drum 3, the rotating heads 2A and 2B simultaneously form an A channel track TA2 and a B channel track TB2, respectively. These operations are alternated by the two pairs of rotating heads. In the above process, two tracks formed simultaneously by each pair of rotating heads, 1A and 1B or 2A and 2B, constitute one segment. Recorded in each segment is a video signal and a corresponding pulse code modulated (PCM) audio signal for a half-field. Therefore, four tracks, for example, TA1, TB1, TA2 and TB2, are used to record one field of signal data, and four segments are employed to record one frame of signal data.
When recording the HDTV signals with the VTR, the video signal, which includes a luminance signal and two color difference signals, is recorded in an analog format, whereas the audio signal, which is a PCM signal, is recorded in a digital format over four channels each representing 16 bits of audio data. These recorded video and audio signals are accommodated in different recording areas, which are arranged longitudinally, of the magnetic tape 4.
Experience has shown that providing a plurality of video signal recording areas per track complicates video signal processing. If the video recording area per track is therefore not divided, then there are three ways of establishing video and audio signal recording areas per track, as shown in FIGS. 9 (A), 9 (B) and 9 (C). In these figures, VA identifies a video signal recording area and PA identifies a PCM audio signal recording area.
In the arrangement of FIG. 9 (A), a PCM audio signal recording area PA is located on the starting side of each of the tracks TA1, TB1, TA2 and TB2 scanned by the rotating heads 1A, 1B, 2A and 2B, respectively. In the arrangement of FIG. 9 (B), a PCM audio signal recording area is located on the ending side of each track scanned by each head. In the arrangement of FIG. 9 (C), one PCM audio signal recording area is located on the starting side and one area is located on the ending side of each track scanned by each head.
Therefore, in each of the above-described arrangements, a PCM audio signal recording area PA is located on the starting and/or ending side of each head-scanned track. However, the head scan starting or ending part of any track is vulnerable to scratches or damage which may be inflicted because of its location at a tape end. This damage tends to produce drop-outs in the reproduced signal. Furthermore, since the head scan starting part of the tape is the part where a head starts contacting the tape, a relatively snug fit between the head and the tape has not as yet been achieved. This can cause the RF reproduction output to fluctuate and can lead to signal dropouts. This problem is more likely to occur the closer the position of contact is to a longitudinal end of the tape.
When the audio signal is conventionally recorded in the PCM signal format, the so-called interleaving process is utilized and error correction codes are added to the audio signal. The interleaving process generally involves distributing PCM audio data samples within the recording areas according to a predetermined set of rules. To record the audio signal, the audio signal of each channel is distributed to the two tracks of each segment. In interleaving processing, if a data sample from the signal of one of the two tracks has dropped-out and cannot be corrected by an error correction code, then for reproduction, another data sample which is intact and located relatively near the dropped sample is interpolated so as to approximately reconstruct the missing audio signal.
However, signal reconstruction utilizing interpolation techniques, as discussed above, inevitably causes some degree of degradation in the sound quality. The greater the magnitude of signal drop-outs, the more pronounced the sound quality degradation.
If the audio signals of all four channels are recorded in the above-described conventional manner and under the same conditions, then all of the reproduced signals will contain the above-described draw-backs.